In an ink jet printing process, individual drops of dye or pigment are deposited onto a substrate with on demand droplet deposition devices comprising dozens or hundreds of individual nozzles spaced typically {fraction (1/300)} or {fraction (1/600)} inches apart. The image quality possible with color ink jet printers now approaches photorealistic. It can be appreciated that to produce such high quality images, the nozzles on the droplet deposition devices should be functioning properly, and should be depositing droplets precisely onto the desired locations on the substrate. In some cases, a single malfunctioning nozzle out of the hundreds which are depositing droplets can have a noticeable effect on image quality.
A number of different techniques for evaluating nozzle function have been developed. In some systems, the existence and trajectory of the ink droplets is detected as the droplet moves through the air between the nozzle and the substrate. One example of a system of this type is described in U.S. Pat. No. 4,510,504 to Tamai et al. In other systems, droplets are ejected onto the print substrate, and are optically detected from above. This technique is utilized in some commercially available products from, for example, Hewlett-Packard, of Palo Alto, Calif. and ColorSpan Corp. of Eden Prairie, Minn. These detection systems typically include one or more LED light sources and an optical detector mounted on the moveable print carriage. The detector senses LED light reflected from the substrate, and the properties of this reflected light are analyzed. Such designs require the use and disposal of a certain amount of media, which can be very expensive in high quality image production. Furthermore, the accuracy and sensitivity of these systems is greatly impaired when coarse or uneven media such as canvas is being printed.
Another system is shown in U.S. Pat. No. 4,493,993 to Kanamuller et al. In the Kanamuller patent, droplets are deposited onto a rotating transparent disk. The presence of individual droplets is detected by a detector on the other side of the disk. The deposited droplets are wiped off of the disk after detection by passing the disk across an absorbing pad. The Kanamuller system is limited in that certain types of nozzle malfunctions are difficult or impossible to detect. The system of Kanamuller also requires a relatively messy cleaning system. Improved methods of evaluating nozzle functionality are therefore needed in the art.
The invention comprises an inexpensive and fast method of droplet deposition analysis in an ink jet printer. Advantageous apparatus for performing the method is also provided. In one embodiment, an apparatus for droplet deposition analysis comprises a strip of substantially transparent and flexible film. The film may have a light source mounted on one side and an optical detector on the other. The use of flexible film produces a less expensive and more consistent droplet deposition analysis than is found in the prior art.
Advantageous droplet analysis methods provided by the invention include depositing an array of ink droplets onto a transparent substrate, passing light through the transparent substrate and into an optical detector so as to detect said array of ink droplets, mapping the array of ink droplets onto a coordinate field, and detecting at least one ink droplet which is incorrectly placed relative to the coordinate field.
Advantageously, such methods and apparatus are implemented in ink jet printers to produce higher quality print output in a shorter time, and with less material waste.